My doorbell transformers have always been fairly far from the doorbell itself. The electrical panel is in the basement and the doorbell on the main level in the center of the house or close to the front door. It means a longer run of low-voltage 2 conductor cable (little copper) versus a longer run of mains voltage wire (3 conductors, 14ga, a lot more copper).

The central vac thing makes some sense too. The wire would be used for the switching and accessory power. But it still seems like an odd location/placement.

I've never seen a low-voltage garage door opener - and again the placement and wire run for this would be really odd.